(a) Field of the Invention
The present invention relates to a semiconductor device for securely protecting its internal element from static electricity and a method for manufacturing the same, and more in detail to the semiconductor device including the internal element and a protection element in which electrostatic energy is discharged through the protection element for protecting the internal element and the method for manufacturing the same.
(b) Description of the Related Art
In a conventional semiconductor device, protection elements are generally connected to input and output terminals for protecting an internal element against electrostatic breakdown. That is, electrostatic energy is discharged through the protection circuit toward a grounded line (GND) or a power source line VCC) for protecting the internal element before entering the internal element.
Especially, in a bipolar transistor device, the decrease of a parasitic capacitance is essential for a high-speed operation to thereby make a collectors base junction and a base-emitter junction shallower. When the static electricity enters the internal element (bipolar transistor), electrostatic breakdown is likely to occur due to concentration of an electric field. Accordingly, high-performance protection elements have been proposed heretofore.
JP-A-4(1992)-22163 describes a semiconductor device including a conventional protection element. An example of a circuit construction of the conventional semiconductor device (first conventional example) is shown in FIG. 1 and a vertical cross section of the semiconductor device is shown in FIG. 2. The first conventional example employs a reverse discharge as well as a forward discharge of a P-N junction by using a breakdown phenomenon of the P-N junction.
As shown in FIG. 1, the emitter and the base of a bipolar transistor are connected and base current starts to flow when a breakdown voltage is applied between the base and the collector by entering static electricity in the first conventional example. The bipolar transistor is driven for operation by the breakdown current flowing through the base resistance as a trigger. Due to the amplification factor (hfe) of the bipolar transistor as high as to 50 to 150, the discharge path is switched from the base-collector path to the collector-emitter path by the function of the transistor. The bipolar operation completes the discharge at a moment for effectively acting as the protection element. Since a plenty of current flows at a moment, the size of the protection element is required to be larger than the internal transistor for preventing the destruction of the protection element itself.
As shown in FIG. 2, the conventional semiconductor device includes an N-type epitaxial layer 102 overlying a P-type silicon substrate 101, and an N-type embedded layer 103 between the P-type silicon substrate 101 and the N-type epitaxial layer 102. The N-type epitaxial layer 102 on the N-type embedded layer 103 functions as the intrinsic collector region of the bipolar transistor.
A field oxide film 110 is formed by selectively replacing the region other than a diffused region with an oxide film. A P-type dielectric layer 104 underlying the field oxide film 110 functions as an isolation layer for surrounding the side surface of the transistor and has a depth reaching to the surface of the silicon substrate 101.
A base layer 105 is formed as a P-type layer overlying the N-type embedded layer 103, and an N-type emitter layer 106 is formed in the base layer 105.
A heavily doped N-type collector layer 107 is formed between a collector electrode 111 and the N-type embedded layer 103, and a voltage is applied to the N-type embedded layer 103 through the N-type collector layer 107.
A silicon oxide film 108 functions as a dielectric film covering the N-type epitaxial layer 102, and the respective via holes are formed in the silicon oxide film 108 corresponding to the base layer 105, the N-type emitter layer 106 and the N-type collector layer 107.
An aluminum electrode 109 is disposed on the via holes to connect a base electrode and an emitter electrode.
In the bipolar transistor as the protection element in the conventional example 1, the breakdown voltage is determined by a base-collector breakdown voltage which is not largely different from that of the internal transistor (the transistors of the protection element and the internal element have substantially the same structure).
The selective discharge is generally designed to occur by employing the protection element having a long discharge path (or a large size). However, when the base-collector breakdown voltage of the internal transistor becomes lower than that of the protection element due to variation of manufacturing conditions, the discharge of the internal transistor starts earlier than that of the protection element, and the internal transistor is disadvantageously destroyed.
A circuit diagram of a second conventional example which achieves the improvement of the above disadvantage is shown in FIG. 3. A sectional view of a semiconductor in FIG. 3 corresponding to FIG. 2 is omitted because only electrodes to be connected are modified.
In the circuit shown in FIG. 3, the collector and the emitter are reversed from an ordinary circuit, wherein and the collector and the base are connected. A voltage is applied between the emitter and the base, and when a breakdown starts therebetween, a reverse operation of the bipolar transistor is triggered by the breakdown current flowing through a base resistance. Since its amplification factor (reverse hfe) is about 1 which is not high, the current flowing in the discharge path is divided into the breakdown current and the current flowing between the emitter and the collector. The division of the current in the second conventional example shown in FIG. 3 more rapidly completes the discharge.
Further, since the emitter-base breakdown voltage of the protection element is less than half the base-collector breakdown voltage of the internal transistor, the first discharge advantageously starts in the protection element.
However, the reduction of the thickness of the emitter layer in the second conventional example for a high-speed operation increases the density of the breakdown current flowing between the emitter electrode and the base electrode, thereby causing a destruction.
In view of the foregoing, an object of the present invention is to provide a semiconductor device which has a sufficient durability against the destruction and can protect an internal element. Another aspect of the present invention is to provide a method for manufacturing such a semiconductor device.
The present invention provides, in a first aspect thereof, a semiconductor device comprising: a semiconductor substrate having a first conductivity-type; a first embedded layer having a second conductivity-type and formed in the semiconductor substrate; a second embedded layer having a second conductivity-type and formed in the semiconductor substrate, the first embedded layer having a depth larger than a depth of the second embedded layer; an internal bipolar transistor having an emitter, a base and a collector which is formed as the first embedded layer; and a protection bipolar transistor having an emitter, a base and a collector which is formed as the second embedded layer, the base and the emitter of the protection transistor being connected together. In the first aspect of the present invention, the first and the second embedded layers may be modified as long as current is likely to flow in the second embedded layer.
The present invention provides, in a second aspect thereof, a method for manufacturing a semiconductor device including the steps of: forming a first embedded layer having a second conductivity-type and a second embedded layer having the second conductivity-type overlying a semiconductor substrate having a first conductivity-type; forming heavily doped collector layers reaching to each of the first and the second embedded layers; forming intrinsic collector regions overlying each of the first and the second embedded layers such that an electric resistance of the intrinsic collector region overlying the first embedded layer is larger than that of the intrinsic collector region overlying the second embedded layer; forming an external base layer having the first conductivity-type and an intrinsic base layer having the first conductivity-type in each of the intrinsic collector regions; and forming an emitter layer having the second conductivity-type in the intrinsic base layer.
Although the bipolar transistor is employed as the protection element in the first and the second aspects of the present invention, a MOS transistor may be replaced therewith.
In accordance with the present invention in which the electric resistance between the collector and the emitter (in case of bipolar transistor) or between the source and the drain or between the source or the drain and the embedded layer (in case of MOS transistor) of the protection element is higher than that of the internal element to be protected, even if static electricity is generated in the circuit, the static electricity is discharged in the protection element having the lower electric resistance and the internal element is no longer damaged.
The sufficiently larger difference between the electric resistances of the protection element and the internal element can also avoid possible damages of the internal element generated due to variations of junction resistances depending on manufacturing and operating conditions.
The internal element is securely protected by preventing the generation of breakdown current without using an extremely thin emitter layer.
The above and other objects, features and advantages of the present invention will be more apparent from the following description.